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AM Max II Modulation Problem

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AM Max II Modulation Problem
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Joined: 26 Jan 2005
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Post AM Max II Modulation Problem Reply with quote
Just recieved my AM Max II. I ordered it in the enclosure with the 19 V power supply. The carrier seems good and strong, but my modulation level is pitiful.

I don't have any way to accurately mesure it, but just listening to it I would be surprised if I'm above 20%. I've tried various audio sources at various levels. I've tried tweking the compression parameters, specifically decreasing the compression ratio and increasing the threshold, but it makes no audible difference. This leads me to believe that the DSP circuitry is not receiving sufficient levels to make the compression kick in.

I've also tried increasing the modulation trimmer on the board. This makes a slight difference, but incresing it too much just leads to distortion. Not overmodulation--just a very quiet, clipped sound.

Marko's suggestion was to set the threshold higher, but, as noted above, the threshold setting doesn't really do anything.

Any ideas would be appreciated.
Wed Jan 26, 2005 6:53 pm View user's profile Send private message
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Post Did you download the manual? Reply with quote
Besides the frequency, there are seven settings...

Compression threshold
Compression level
Integration interval
Low pass filter

It sure would be nice if the manual would fully explain what these nebulous settings are used for and why they exist. A better job of tech writing would save a lot of questions and confusion. It took me about 3 weeks of farting around to get the right sound...
Fri Jan 28, 2005 3:18 am View user's profile Send private message
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Post Reply with quote
Let me try to explain.... Someone let me know if I get this wrong....

Treble - high frequency emphases

Compression threshold - the audio level at which compression occurs

Compression level - the ratio of compression - Less is better for crystal clear audio( I use between 3 and 4 max. Using more compression and then more audio could help you keep your carrier fullly modulated (100%). I use a couple of devices to keep my carrier fully modulated (level).. a compellor to level the input to a suitably high level and before that, software, "mp3gain" to level my program material. (I hope this understandable)

Attack - the quickness that compression occurs once the threshold has been exceeded

Decay - the time over which which compression is phased out

Integration interval - the rate at which the amplitude of the audio is sampled so that instructions can be sent to the compression circuitry.

Low pass filter - Typically A RF filter that filters RF outside the desired frequency you are transmitting on. (ie, harmonic frequencys)

In the case of the "AM Max," I think the meaning of this term is that of what I have always called a band pass filter. Ie, 4500 kc X 2 (upper and lower sideband) equals a carrier which is 9 kc wide, which is less than the 10 kc band width for an AM signal.

Fri Jan 28, 2005 11:07 pm View user's profile Send private message
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Post Nice Try... Reply with quote
It's a respectable try, Antler.

Couple of things;

Your stumped like me when it comes to the Integration Interval...What is it?

LPF: The lower and upper sidebands (4500 Hz) would fit in the European standard (9 kHz) of band spacing. Here the States the channel spacing is 10 kHz. Going up to 7500 would probably give greater fidelity but splash into adjacent channels pissing everyone off. I know that around the house I got to lower receiver gain or I'm heard all over the dial.

A word about harmonics, I take it your talking about audio harmonics which give a sound realism. RF harmonics is a different story, that's a natural product. I'm at 1640 kHz fundamental but I also transmit at 3280, 4920 and so on for maybe couple hundred feet.

So anyhow, how do you like your unit? How is that EH antenna going?
Sat Jan 29, 2005 3:15 am View user's profile Send private message
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Post Reply with quote
Hi guys!

Well, 9KHz and 10KHz are pretty close and since I was
running out of space in that tiny little microcontroller,
I had to make some choices. I guess using 4500 instead
5000 is pretty much the same, especially if you add a bit
of treble in the mix.

I wanted to include 7500KHz for those asking for high fidelity.
Also wanted to include pass all and 3300 for hams.

You figured out the rest of the stuff quite well. But the new
manual on the web covers this too, no?

Best regards,
Marko - PCS Electronics
Turn your PC into a FM radio station!
fax +386 4 2316 128
Sat Jan 29, 2005 11:11 am View user's profile Send private message Visit poster's website
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Post Reply with quote
Well, on Integration Interval - I would call this "sample rate or time interval." You will see the term "sample rate" used a lot in discussion of audio DSP in the specifications of various types of digital recording equipment and etc. Sample rate would be defined as the frequency or amount of time the DSP takes off between "snapshots." In the case of the AM max, the frequency the DSP uses to see if the incoming signal is too loud, ie, if too loud, the compressor is told to do some smoothing or compression of the incoming signal.

BTW, In some DSP circuits, beside compression, it is also possible to add gain to help smooth out the incoming signals.

On the EH, I just finished a ten day road trip and will have to go back out pretty quick. I don't see me having time to work on it for a couple of weeks. I tried to pay a couple of diffenent guys to build one for me and they were both way out of my ball park. One of the guys is considering the making a few antennas to sell commercially. If he does, the price should be around $200.

One problem with the EH is that the antenna, to be really effective, needs to be up around 77 feet for 1600 to 1700 kc band. That would pretty hard for me achieve. On a good day I might get it up 30 to 45 feet but there is no way for me to get it up as high as it really needs to be to be effective. There is a commercial station in Tucon, I understand, that is using an EH antenna (10 KW) with very good results.

My comments on harmonics were indeed on RF harmonics not audio but just as you described them.

On the "pass band filter" or LPF, 4500 kc will work just fine for me. My ears won't hear much better than that and I know that I will stay within the 10 kc US standard for band width. The downside of being wider that 10 kc is that on good radios (like my Yacht boy receiver and other receivers of similar quality) the pass band is set to 10 kc and the audio will sound clipped or distorted if the signal is wider than that.

I guess I will try to get up a long wire antenna or something like it today just to see how this thing works. With the AM max on my dummy load I am getting out 5 or 600' and the audio sounds typical for AM.

Sat Jan 29, 2005 11:25 am View user's profile Send private message
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